In process industry, there is a need to filter suspensions containing liquid and solid matter particles for the purpose of sampling. For instance in paper and pulp industry, filtrate samples are taken from stock for the purpose of monitoring and controlling the process. Stock contains liquid, small amounts of gas, and solid matter particles, such as fibres and possibly slivers. When it is necessary to measure the properties of the liquid in the stock, the solid matter particles are filtered from the sample as well as possible. For this, different filtering devices have been developed, such as those described in published patents FI 57663 and SE 511069, incorporated herein as reference.
In the solution disclosed in FI 57663, the sample is filtered with a screen comprising narrow slots. When the sample is taken, the slots prevent the fibre-like particles from advancing, the liquid flows on to the measurements. When the screen is cleaned, pressurized air is blown towards the process to return the fibres and slivers stuck to the screen to the process. A big problem is, however, that the fibres and slivers are usually so tightly stuck in the slots of the screen that the cleaning is not entirely sufficient. For this reason, the screen is too quickly blocked and needs to be changed. A blocked screen produces a sample slowly.
SE 511069 discloses a sampler comprising a screen which has a slotted surface made of triangular wire. In this screen, too, the fibres and slivers remain in the slots while the liquid flows to the measurement. The screen is cleaned by scraping. This solution, too, has the same problems as that described above. The cleaning is not entirely sufficient, the screen is blocked quite quickly by fibres and slivers and needs to be changed often. A blocked screen produces a sample slowly.